Setting $RT::LogToScreen doesn't change logging

While RT->Config->Set('LogToScreen','error') sets $RT::LogToScreen for
backcompat, we don't read from $RT::LogToScreen when calling
RT->Config->Get('LogToScreen');

This exposes a number of suppressed warnings when running rt-serializer
against 4.2-trunk.

    LogToScreen is deprecated for LogToSTDERR.
    Case sensitive search by Groups.Domain
    Use of RT::Tickets::_SQLLimit in Queue.pm

Since parts of this need to go back into rtx-migrate and we need to
maintain backcompat in rt-serializer as much as possible (for
incremental upgrades) cleanup of these will come in separate commits.